1. Field of the Invention
The present invention relates to a speaker unit, a speaker system comprising such a speaker unit, and a method for manufacturing a speaker diaphragm.
2. Description of the Related Art
Light-weight materials with high modulus and substantial internal loss are considered suitable for speaker diaphragms. Materials based on pulp starting materials have been most frequently used conventionally as speaker diaphragms because of their light weight and their suitable internal loss. However, with recent concerns over the environmental destruction resulting from deforestation, even more light-weight materials with high modulus have been developed. Many speaker diaphragms of synthetic resins, in particular, have been developed, and it is known that weight can be reduced and that the modulus can be improved by using synthetic resins and foams such as polystyrene or polypropylene, as well as composites of such synthetic resins or synthetic resin foams and other materials, for such diaphragms.
New speaker units are required to have higher amp output and to withstand use in a variety of environments such as car stereo speaker units which are used under oppressive conditions involving changing temperatures. Speaker diaphragms should therefore have high heat resistance and undergo fewer changes in acoustic properties due to changes in temperature. However, even though polystyrene resins have high modulus and are inexpensive, their heat resistance is problematic, and a resulting problem is that diaphragms made of polystyrene resin suffer considerable loss of modulus at elevated temperatures. Although polypropylene resins are crystalline resins with a relatively high melting point, the resin properties are considerably temperature-dependent. A resulting problem is that changes in temperature can cause changes in the acoustics of diaphragms made of polypropylene resin. Additionally, an advantage of diaphragms comprising polystyrene resin or polypropylene resin foams is that they weigh less than diaphragms made of unfoamed synthetic resins, yet the rigidity is lower.
In view of the foregoing, an object of the present invention is to provide a better speaker diaphragm that weighs less, that has higher rigidity and modulus, and that has fewer changes in properties as a result of changes in temperature, as well as a speaker unit featuring the use of such a diaphragm, and a speaker system comprising such a speaker unit.
The speaker unit of the present invention is a speaker unit comprising a diaphragm and a mechanism for driving the diaphragm, the aforementioned diaphragm comprising a 0.5 to 10 mm thick aromatic polycarbonate resin foam with a density of 0.03 to 0.6 g/cm3. The diaphragm of the speaker unit in the present invention comprises the steps of foaming an aromatic polycarbonate resin foam sheet or plate, and the elastic modulus in tension of said foam sheet or plate is preferably at least 1 MPa at temperatures of 25xc2x0 C., 50xc2x0 C., 80xc2x0 C., and 105xc2x0 C., and the tan xcex4 of said foam sheet or plate is preferably at least 0.02 at a temperature ranging from 25 to 105xc2x0 C. during measurement of the dynamic viscoelasticity in bending tests giving 1 Hz frequency oscillation strain. The diaphragm comprises the steps of foaming an aromatic polycarbonate resin foam sheet or plate, and the mean cell diameter of said foam sheet or plate is preferably 0.05 to 1 mm, and the amount of foaming agent left over in the foam forming said diaphragm is preferably no more than 0.3 mol/kg. The aromatic polycarbonate resin forming the diaphragm in the speaker unit of the present invention preferably comprises an aromatic polycarbonate resin derived from bisphenol, and an aromatic polycarbonate resin with a viscosity average molecular weight of 25,000 to 70,000 is preferred. The percentage of open cells in the aromatic polycarbonate resin foam forming the diaphragm in the speaker unit of the present invention is preferably no more than 50%. The diaphragm of the speaker unit in the present invention can comprise a film or sheet of an unfoamed thermoplastic resin laminated to at least one side. The speaker system of the present invention comprises a speaker unit as described above attached to a cabinet. The method for manufacturing a speaker diaphragm in the present invention comprises the steps of forming a 0.5 to 10 mm thick aromatic polycarbonate resin foam sheet or plate with a density of 0.03 to 0.6 g/cm3.